This invention relates to cathode ray picture tubes used in projection television systems, and is particularly concerned with means and method for optimizing the performance of such tubes.
Projection television systems typically include at least one cathode ray picture tube having a cathodoluminescent screen on the inside surface of the face panel. Electron-beam generating means disposed on the cathode ray tube electron-optical axis provide for forming an electron image on the cathodoluminescent screen. This electron image is converted to a visible image by the screen. Projection lens means on the projection optical axis of the tube provide for projecting the aerial image of the visible image onto a viewing screen whereon the viewer sees the television picture. The viewing screen may be of the front-projection type, or a rear projection type wherein the aerial image is projected onto the side of the screen opposite the viewer. To provide for compactness of the projection system, the path of the aerial image is normally "folded" by means of one or more mirrors.
Projection television systems may have a bank of red, green and blue image source means including three cathode ray picture tubes each with an associated projection lens for projecting into coincidence a red, green, and blue image to form a composite color image on the viewing screen.
A desirable--indeed necessary--feature of a projection television system is the ability to project an image of adequate brightness on the viewing screen. Brightness, desirably, should equal that of the shadow mask color picture tube which provides an average brightness of 80 foot-Lamberts at a beam current of 1.5 milliamperes, with a peak brightness potential of about 320 foot-Lamberts. In view of the relatively long projection path and consequent effect of the inverse-square law, this brightness objective has proved difficult to achieve in projection television systems.
The face panel of a cathode ray picture tube used in projection television systems is typically circular, with a diameter of about six inches. The visible image that is electron-formed on the cathodoluminescent screen on the inner surface of the face plate is a rectangle of three to four aspect ratio. To provide a projected image of four feet in diagonal measure having a brightness of eighty foot-Lamberts, for example, the brightness of the image on the cathodoluminescent screen must be in the range of six thousand to seven thousand foot-Lamberts.
An undesired byproduct of image brightness of this magnitude is the undesirably high temperatures which are developed in the envelope, especially in the face plate area, as a result of the electron bombardment of the face plate. For example, the operating temperature of the face plate may vary between 80 degrees Centigrade and 90 degrees Centigrade with 80 degrees Centigrade being considered a practical maximum. As the cathode ray tube envelope is made of glass, the envelope is prone to thermal cracking, especially in the area of the imaging screen of the face plate.
One way to obviate the thermal cracking tendency is to reduce the power consumption of the cathode ray tube from about sixteen watts, for example, which provides a projected picture of acceptable brightness, to about eight watts, wherein the projected picture becomes of marginally acceptable brightness. This obviously self-defeating measure has proved necessary in some prior art projection systems to provide acceptable reliability.
So a major factor in limiting the brightness of a projection television system is the thermally induced cracking of the CRT face plate. Another factor is the need to make the glass of the face plate thicker to provide greater crack resistance, whereupon the thicker glass appreciably reduces light transmission and thus makes for a less bright image.
Even if the face plate does not crack as a result of heat, the high temperatures concentrated in the small imaging area can produce other undesirable results. For example, high temperatures coupled with electron bombardment over a period of time can cause the glass to discolor, which in turn can degrade the color purity of the projected image. Another deleterious effect of high face plate temperature is exerted on the associated projection lens, which is normally located closely adjacent to the face plate. The composition of such lenses is usually a plastic which provides light weight and lower costs; however, the plastic is heat deformable. If the face plate temperature reaches 100 degrees Centigrade, however, the plastic may deform and destroy the lens.